Yangzi Qiao

Cavitation distribution within large phantom vessel and mechanical damage formed on surrounding vessel wall

Blood vessel is one of the most important targets encountered during focused ultrasound (FU) therapy. The lasting high temperature caused by continuous FU can result in structural modification of small vessel. For the vessel with a diameter larger than 2mm, convective cooling can significantly weaken the thermal effect of FU. Meanwhile, the continued presence of ultrasound will cause repetitive cavitation and acoustic microstreaming, making comprehension of continuous wave induced cavitation effect in large vessels necessary. The Sonoluminescence (SL) method, mechanical damage observation and high-speed camera were used in this study to investigate the combination effect of ultrasound contrast agents (UCAs) and continuous FU in large phantom vessels with a diameter of 10mm without consideration of thermal effect. When the focus was positioned at the proximal wall, cylindrical hole along the acoustic axis opposite the ultrasound wave propagation direction was observed at the input power equal to or greater than 50 W. When the focus was located at the distal wall, only small tunnels can be found. The place where the cylindrical hole formed was corresponding to where bubbles gathered and emitted brilliant light near the wall. Without UCAs neither such bright SL nor cylindrical hole can be found. However, the UCAs concentration had little influence on the SL distribution and the length of cylindrical hole. The SL intensity near the proximal vessel wall and the length of the cylindrical hole both increased with the input power. It is suggested that these findings need to be considered in the large vessel therapy and UCAs usage.

Sonochemiluminescence observation and acoustic detection of cavitation induced by pulsed HIFU at a tissue–fluid interface

The aim of this study is to investigate the mechanism of the erosion process induced by 1.2 MHz pulsed high-intensity focused ultrasound (pulsed HIFU). By using Sonochemiluminescence (SCL) photograph, the initiation and maintenance of active cavitation were observed. In order to understand the role of both inertial cavitation and stable cavitation, a passive cavitation detection (PCD) transducer was used. Since the exposure variables of HIFU are important in the controlled ultrasound tissue erosion, the influence of pulse length (PL) and duty cycle (DC, Ton:Toff) has been examined. The results of tissue hole, SCL observation and acoustic detection revealed that the erosion was highly efficient for shorter PL. For higher DCs, the area of SCL increased with increasing PL. For lower DCs, the area of SCL increased with increasing PL from 10 to 20 μs and then kept constant. For all PLs, the intensity of SCL decreased with lower DC. For all DCs, the intensity of SCL per unit area (the ratio of SCL intensity to SCL area) also decreased with increasing PL from 10 to 80 μs, which suggested that the higher the intensity of SCL is, the higher the efficiency of tissue erosion is. At DC of 1:10, the position of the maximum pixel in SCL pictures was distant from the tissue-fluid interface with the increasing PL because of shielding effect. By the comparison of inertial cavitation dose (ICD) and the stable cavitation dose (SCD), the mechanisms associated with inertial cavitation are very likely to be the key factor of the erosion process.

Sonochemiluminescence observation of lipid- and polymer-shelled ultrasound contrast agents in 1.2 MHz focused ultrasound field

Ultrasound contrast agents (UCAs) are frequently added into the focused ultrasound field as cavitation nuclei to enhance the therapeutic efficiency. Since their presence will distort the pressure field and make the process unpredictable, comprehension of their behaviors especially the active zone spatial distribution is an important part of better monitoring and using of UCAs. As shell materials can strongly alter the acoustic behavior of UCAs, two different shells coated UCAs, lipid-shelled and polymer-shelled UCAs, in a 1.2 MHz focused ultrasound field were studied by the Sonochemiluminescence (SCL) method and compared. The SCL spatial distribution of lipid-shelled group differed from that of polymer-shelled group. The shell material and the character of focused ultrasound field work together to the SCL distribution, causing the lipid-shelled group to have a maximum SCL intensity in pre-focal region at lower input power than that of polymer-shelled group, and a brighter SCL intensity in post-focal region at high input power. The SCL inactive area of these two groups both increased with the input power. The general behavior of the UCAs can be studied by both the average SCL intensity and the backscatter signals. As polymer-shelled UCAs are more resistant to acoustic pressure, they had a higher destruction power and showed less reactivation than lipid-shelled ones.

Sonoluminescence and sonochemiluminescence study of cavitation field in a 1.2MHz focused ultrasound

An intensified CCD (ICCD) and an electron-multiplying CCD (EMCCD) were employed to observe the spatial distribution of sonoluminescence (SL) and sonochemiluminescence (SCL) generated by cavitation bubbles in a 1.2MHz HIFU field. Various sonication conditions, which are free field and focal region near a water-parenchyma interface, were studied. In addition, the differences of two shells coated UCAs were also investigated. In this study, an acoustic radiation force (ARF) counterbalance appliance was added to reduce bubble displacement. Cavitation mapping in this situation was also operated through SCL recording. SCL was also employed to measure cavitation does and map the spatial distribution of cavitation near a boundary of parenchyma.

Phase-shift nano-emulsions induced cavitation and ablation during high intensity focused ultrasound exposure

Phase-shift Nano-emulsions (PSNEs) with a small initial diameter in nanoscale have the potential to leak out of the blood vessels and to accumulate at target point of tissue. At desired location, PSNEs can undergo acoustic droplet vaporization (ADV) process, change into gas bubbles and enhance focused ultrasound efficiency. The aim of this work was to provide spatial and temporal information on PSNE induced cavitation and ablation effects during pulsed high intensity focused ultrasound (HIFU) exposure. The PSNEs were composed of perfluorohaxane (PFH) and bovine serum albumin (BSA), and then uniformly distributed in a transparent polyacrylamide phantom. The Sonoluminescence (SL) method was employed to visualize the cavitation distribution and formation process of PSNEs induced cavitation. For the phantom which was used for ablation observation, heat sensitive BSA was added. When the temperature generated by ultrasound exposure was high enough to denature BSA, the transparent phantom would turn out white le...

Fundamentals of Cavitation

Cavitation is defined as the formation of one or more cavities in a liquid. The word “formation” can refer, in a general sense, to both the creation of a new cavity and the expansion of a preexisting gas pocket to a size where macroscopic effects, e.g., its shape and size, acoustic emissions, sonoluminescence, and erosive properties, can be observed. The cavity’s gas content refers to the liquid’s vapor, gas dissolved in the liquid, or combinations thereof. Cavitation usually occurs as a response when the pressure has been reduced sufficiently below the vapor pressure of the liquid or when the temperature has been elevated above the boiling point. In addition, chemical-, electrical-, and radiation-induced cavitation also exists. This book only considers the first case of cavity formation—that associated with a drop in pressure or a rise in temperature—in response to an acoustic field.

Cavitation Control and Applications

Cavitation is an important phenomenon that can occur both during diagnostic and therapeutic applications. It is not only an indicator of the transient high pressure and temperature induced by intense bubble collapse, but it also mediates mechanical and thermal effects. As far as mechanical effects, collapsing bubbles can emit shock waves and liquid jets, which are considered the main mechanisms that cause mechanical damage (Kodama and Tomita in Appl Phys B 70:139–149, 2000).

Study of Ultrasound Contrast Agents in 1.2MHz Focused Ultrasound Field Based on Sonochemiluminescence

Focused ultrasound (FU), especially high intensity focused ultrasound (HIFU) is a very promising therapeutic method due to its non-invasive and efficient character. Ultrasound contrast agents (UCAs) are frequently added into HIFU field as cavitation nuclei to enhance the therapeutic efficiency. However, their presence will distort the pressure field, shield the initial target and make the process unpredictable. In order to better monitor and use UCAs, their behavior, especially the active zone spatial distribution in ultrasound field must be understood.